Chapter Flood Consequences

Transcription

1 Chapter Flood Consequences

2 438

3 16. Flood Consequences Introduction and Scope of Topic This chapter identifies and describes the existing flood risk features along the route of the Scheme and then assesses the significance of impacts that could occur as a result of the implementation of the scheme. New development in the floodplain can have implications for flooding both of the new development itself and elsewhere. The proposed Scheme impacts on both Main River and Ordinary Watercourses, namely the Ashton/Longmoor Brook, Colliter s Brook, the Malago, and a series of small watercourses south of the A Effects on the water environment in relation to flood risk during both the construction and operation phase are identified and assessed. The assessment methodology followed is in accordance with the guidance provided in the Design Manual for Roads and Bridges (DMRB) Volume 11 Section 3 Part 10. Design Principles The following table outlines the key design principles relating to flood risk for the proposed Scheme, these were agreed with the Environment Agency. Table 16.1: Design Parameters Parameter Specification Design Life 60-years (to 2072) Climate Change Impacts Sea Level Rise 481mm over 60 year to 2072 (TGNPPF Table 4 (South West Region)) Rainfall Intensity 30% increase in peak rainfall intensities River Flow 20% increase in peak flow (TGNPPF Table 5 (period 2055 to 2085)) Flood Risk Management Standard of Protection New Works Tidal 0.5% AEP (1 in 200-years) Fluvial 1% AEP (1 in 100-years) including climate change allowance (and 0.6m freeboard) Standard of Protection Existing infrastructure Improvement where feasible Compensatory Flood Storage Up to 1% AEP (1 in 100-years) including climate change allowance Environment Agency Maintenance Where possible improve, or as a minimum maintain Environment Agency maintenance access arrangements Surface Water Management New works 3.3% (1 in 30-year) for surface drainage system. Re-infiltration to ground using SUDS, filter drains, swales, wetland attenuation etc where feasible On existing road network Run off rates to be attenuated to existing rates using storage within scheme boundaries before discharge 439

4 16.2. Policy Context and Guidelines of Relevance The water environment in the UK is protected by legal and planning frameworks. This ensures that proposed and existing activities do not compromise water resources as well as the natural environment. The key legislation and policies relating to flood risk management are outlined below. Water Resources Act Water resource management within the UK is underpinned by the Water Resources Act Under this act the Environment Agency (EA) have the following powers, duties and responsibilities: To form and maintain water management schemes; To impose minimum acceptable flow, level or volume conditions on watercourses and waterbodies; To monitor watercourses, restrict abstractions and impose drought orders; To consent discharges into inland freshwater, groundwater, estuaries and the sea; To identify the extent of flood plains, washlands and other land liable to flood and minimise flood risk; To control and license works undertaken in, over or under any watercourse; To regulate fisheries; and To undertake works or action to remove, dispose or remediate polluting matter in order to restore the state of ecological and chemical status of a watercourse The environmental assessment as a matter of policy adherence would seek to ensure that the construction and operation of the Scheme does not significantly increase the likelihood of water pollution issues within local watercourses. The Scheme as a minimum should aim to minimise flood risk wherever feasible and ameliorate any polluting matter to adhere to this regulation. Controls are also placed on abstracting and/or impounding water under the Water Resources Act as well as controls on discharges that may affect the quality of controlled waters. National Planning Policy Context At the National Planning Policy level, there are two Planning Policy Statements that are directly relevant: Planning Policy Statement 23: Planning and Pollution Control; and Planning Policy Statement 25: Development and Flood Risk. Although now superseded by the National Planning Policy Framework (NPPF), the policy principles remain unchanged and supporting Technical Guidance and associated Technical Guide remains in place. National Planning Policy Framework Previously, the now superseded Planning Policy Statements (PPS) set out the Government s national policies on different aspects of spatial planning in England. Planning Policy Statement 25: Development and Flood Risk, and its accompanying practice guide, set out the Government s spatial planning policy on development and flood risk. It aimed to ensure that flood risk is taken into account by all relevant statutory bodies from regional to local authority planning departments to avoid inappropriate development in areas at risk of flooding and to direct development away from areas of highest risk. Where new development is, exceptionally, necessary in such areas, Government policy aims to make it safe, without increasing flood risk elsewhere and, where possible, reducing flood risk overall The government has reviewed planning policy and released the new National Planning Policy Framework (NPPF) and an accompanying Technical Guide, which supersede PPS25 but retain many of the previous policies. 440

5 Local authorities should only consider development in flood risk areas as appropriate where informed by a site-specific FRA, based upon the Environment Agency s Standing Advice on flood risk. The FRA should identify and assess the risks of all forms of flooding to and from the development and demonstrate how flood risks will be managed to that the development remains safe through its lifetime, taking climate change into account The NPPF and the accompanying Technical Guide assigns the level of risk depending on the annual probability of fluvial flooding occurring as follows: a. Flood Zone 1: Low Probability (<0.1% AEP fluvial / sea flooding); b. Flood Zone 2: Medium Probability ( % AEP fluvial / % AEP sea flooding); c. Flood Zone 3a: High Probability ((>1% AEP fluvial / >0.5% AEP sea flooding); and d. Flood Zone 3b: Functional Floodplain (>5% AEP or designed to flood in 0.1% event) Development should be directed as far as is practicable towards Flood Zone 1 areas to avoid fluvial flood risks wherever this is possible. Any development greater than 1 hectare requires a FRA to address design issues related to the control of surface water runoff and climate change, as well as considering any other potential sources of flood risk for the development site. Vulnerability Classification The vulnerability of the development, or land use, must be taken into account as the consequences of flooding may not be acceptable for particular types of development. The NPPF Technical Guide defines the Flood Risk Vulnerability classification (Table 2 in the NPPF Technical Guide) based on the intended use of a proposed development site. Sequential Test The NPPF states that the risk-based Sequential Test should be applied at all stages of planning. Its aim is to steer new development to areas at the lowest probability of flooding. Development should be directed to areas within Flood Zone 1 wherever possible and, if this is not possible, then sequentially direct development to areas least at risk within Flood Zones 2 and 3. Table 16.2: Design Parameters Flood Risk Vulnerability and Flood Zone Development Compatibility Flood Zone Essential Infrastructure Water Compatible Highly Vulnerable More Vulnerable Less Vulnerable Flood Zone 1 Flood Zone 2 Flood Zone 3a Flood Zone 3b Exception test required Exception test required Exception test required Exception test required Key: Development is not appropriate; Development is appropriate Exception Test If, following the application of the Sequential Test, it is not possible, or consistent with wider sustainability objectives, for the development to be located in zones of lower probability of flooding (i.e. Flood Zone 1) the Exception Test can be applied. This test is only appropriate for use when there are large areas in Flood Zones 2 and 3, where the Sequential Test alone cannot deliver acceptable sites but where some continuing development is necessary for wider sustainable development reasons. 441

6 For the Exception Test to be passed, according to the NPPF the development has to comply with the following: Demonstrate that the development provides wider sustainability benefits to the community which outweigh the flood risk, informed by a SFRA where one has been prepared; and A site-specific FRA must demonstrate that the development will be safe for its lifetime, taking account of the vulnerability of its users, without increasing flood risk elsewhere and, where possible, will reduce food risk overall. Local Planning Policy Local Authorities are required to compile Local Development Frameworks (LDFs), to describe how planning policy is going to be delivered in a local area. LDFs have to take account of flood risk and this is facilitated by commissioning Strategic Flood Risk Assessments (SFRA). These strategic reports quantify the flood risk of the land within the local area. Flood Defence Consent The Environment Agency (EA) has general supervisory duties on all matters relating to flood defence. In order to carry out this role, anyone who intends on carrying out works affecting a watercourse, as described below, must obtain their consent before starting work. The reason for this is to ensure that any works do not endanger life or property by increasing the risk of flooding or cause harm to the water environment For Main Rivers under the Water Resources Act 1991 (Section 109) no person shall, without consent in writing: shall erect any structure in, over or under a watercourse which is part of the main river except with the consent of and in accordance with plans and sections approved by the Authority; carry out any work of alteration or repair on any structure in, over or under a watercourse which is part of main river if the work is likely to affect the flow in the watercourse or impede any drainage works; and erect or alter any structure designed to contain or divert the floodwaters of the main river except with the consent of and in accordance with plans and sections approved by the Authority For Ordinary Watercourses under the Land Drainage Act 1991 (Section 23) no person shall, without consent in writing: erect any mill dam, weir or other like obstruction to the flow of any ordinary watercourse or raise or otherwise alter any such obstruction; or erect any culvert that would be likely to affect the flow of any ordinary watercourse or alter any culvert in a manner that would be likely to affect any such flow From April 2012 Lead Local Flood Authorities (LLFA) have taken on responsibility for providing Flood Defence Consent and enforcement on Ordinary Watercourses. The proposed scheme impacts on both Main River and Ordinary Watercourses hence consent will need to be requested from both the EA and the LLFA for different elements of the scheme. 442

7 16.3. Assessment Methodology Introduction The assessment of flood risk will be based upon methodology from HD45/09 as described below. The assessment has been completed in conjunction with the Flood Risk Assessment (FRA). The significance of potential effects will be determined by: Importance of the receptors; Magnitude of the impacts; and Influence of mitigation measures. Data Sources Consultation with the Environment Agency, Bristol City Council (BCC), North Somerset (NSC) and Halcrow in their role as designers have been a key part of defining the scope of work investigating drainage and hydrology, and has guided the production of the FRA. The following information was collected for the FRA: Environment Agency (EA) Flood Maps covering the site and adjacent areas; Details of the proposed development; LiDAR (Light Detection and Ranging) data; SBL Drainage Strategy (Halcrow, 2013); Ashton Vale to Temple Meads (AVTM) FRA and hydraulic model; Bristol Central Area Flood Risk Assessment (CAFRA) hydraulic model; Topographical survey of the Colliter s Brook upstream of the railway line; Topographical survey of the Viridor landfill site; Records of historic flooding; and Strategic Flood Risk Assessments (SFRA) for NSC and BCC areas. DMRB Methodology Prediction and evaluation of effects follows the requirements of the DMRB detailed assessment process as set out in DMRB Volume 11 Section 3 Part The DMRB methodology starts with identification of the importance of the environmental attribute. The magnitude of impact of the scheme on the attribute is then determined using historical records, calculations and tests from the DMRB, taking into consideration the influence of mitigation measures. The combination of the importance of an attribute and the magnitude of impact on that attribute gives a significance of potential effect on the water environment. Potential impacts of construction and operation have been investigated in accordance with DMRB Volume 11 Section 3 Part Calculations and assessment relevant to an Environmental Statement will be undertaken in accordance with DMRB methodology as follows: Method C: focuses on groundwater effects. This is the standard method for assessing the impact of a scheme on groundwater. Typically this considers the risk of pollution to groundwater of discharges from a scheme. However, the drainage philosophy proposed for this scheme includes no direct discharge to groundwater. The only other potential pathway for the Scheme to impact groundwater would be through a physical intersecting of groundwater levels against new Scheme levels. Method E and F focuses on flood risk. A full FRA has been produced following the guidance of Method E. 443

8 Flood Risk Assessment The approach for the scheme FRA was developed in accordance with methodology set out in DMRB Volume 11 Section 3 Part 10 Annex I Method E. In summary, the salient points are: The FRA has considered flood risk from all potential sources, including combined fluvial / tidal events, pluvial, groundwater and reservoir failure; Hydrological and hydraulic modelling has been undertaken to assess the baseline conditions and with scheme impacts, along with potential mitigation measures. Calculations have been undertaken of the hydraulic capacity of the existing river network including river channels, culverts and structures. These have largely been based on existing hydrological and hydraulic models, namely the Ashton Vale to Temple Meads (AVTM) ISIS model, and the Central Area Flood Risk Assessment (CAFRA) ISIS-TUFLOW model, provided by Arup and BCC respectively. This approach was agreed with the Environment Agency; The level of hydrological and hydraulic assessment has been undertaken in proportion to the perceived level of flood risk in the area. Thus, limited modelling has been undertaken for the small watercourses and field drains south of the A38 and on Highridge Common, as the level of flood risk from these channels is considered to be low; and The FRA has included an assessment of the combined impacts on flood risk of the AVTM route and Ashton Gateway (Bristol City Stadium) proposals, to ensure cumulative impacts are considered. As part of this assessment high level mitigation options have been tested to ensure there is no detrimental impact on flood risk. Assessment Criteria The significance of effects on water environment attributes, such as watercourses, has been determined using the four tables in DMRB Volume 11 Section 3 Part 10 that are shown in Table 16-3 and Table The assessment of significance of potential effects of the Scheme has been made in three stages; firstly, evaluating the importance of a water attribute, secondly, assessing the potential magnitude of impact of the Scheme, with and without mitigation and, thirdly, using a combination of the importance and the magnitude, the significance of potential effect on the water environment has been determined An evaluation has been made of the importance of the water environment by considering relevant features adjacent the Scheme study area and the importance of these features has then been graded using the examples given in Table Examples for assessing the magnitude of impacts are shown in Table A matrix for determining significance of effects is shown in Table 16-5 and examples of the potential significance of effects are shown in Table The modelling completed as part of the FRA has been used to inform the evaluation of attributes. Table 16.3: Evaluating the Importance of Water Environment (flood risk) attributes Importance Criteria Typical Example for Flood Risk Very High Attribute has a high quality and rarity on regional or national scale Flood plain or defence protecting more than 100 residential properties from flooding. High Medium Low Attribute has a high quality and rarity on a local scale Attribute has a medium quality and rarity on a local scale Attribute has a low quality and rarity on a local scale Source: DMRB Volume 11 Section 3 Part 10 Table A4.3 Flood plain or defence protecting between 1 and 100 residential properties or industrial premises from flooding. Flood plain or defence protecting 10 or fewer industrial properties from flooding. Floodplain with limited constraints and a low probability of flooding of residential and industrial properties 444

9 Table 16.4: Assessing the Magnitude of Impact on Water Environment (flood risk) attributes Magnitude Criteria Typical Example for Flood Risk Major Results in loss of attribute and/or quality and integrity of the attribute Increase in peak flood level of (1% annual probability)>100mm (Methods E and F). Moderate Minor Negligible Minor Beneficial Moderate Beneficial Results in effect on integrity of attribute or loss of part of attribute Results in some measurable change in attributes quality or vulnerability Results in effect on attribute but of insufficient magnitude to affect the use or integrity Results in some beneficial effect on attribute or a reduced risk of negative effect occurring Results in moderate improvement in attribute quality Increase in peak flood level of (1% annual probability)>50mm. Increase in peak flood level (1% annual probability)>10mm. Change in flood peak (1% annual probability) <+/- 10mm. Reduction in peak flood level (1% annual probability) >10mm. Reduction in peak flood levels(1% annual probability) >50mm. Major Beneficial Results in major improvement of attribute quality Adapted from Source: DMRB Volume 11 Section 3 Part 10 Table A4.4 Reduction in peak flood levels (1% annual probability)>100mm. Table 16.5: Determining the Significance of Effect on Water Environment (flood risk) attributes Importance of Magnitude of Impact Attribute Negligible Minor (- /+) Moderate (- /+) Major (- /+) Very High Neutral Moderate or Large Large/Very Large Very large High Neutral Slight or Moderate Moderate/Large Large/Very Large Medium Neutral Slight Moderate Large Low Neutral Neutral Slight Slight Moderate Source: DMRB Volume 11 Section 3 Part 10 Table A4.5 Table 16.6: Examples of Overall Significant of Effect Effect Description Very Large Large Moderate Where the proposal would result in degradation of the water environment, because it results in predicted very significant adverse impacts on at least one water attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: An increase in peak flood levels (1% annual probability) > 100mm increasing the risk of flooding to > 100 residential properties. Where the proposal would result in a degradation of the water environment, because it results in predicted highly significant adverse impacts on a water attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: An increase in peak flood levels (1% annual probability) > 50mm increasing the risk of flooding to > 100 residential properties OR an increase of >100mm increasing the risk of flooding to residential properties. Where the proposal may result in the degradation of the water environment, because it results in predicted moderate adverse impacts on at least one attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: An increase in peak flood level (1% annual probability) > 10mm resulting in an increased risk of flooding to > 100 residential properties OR an increase of > 50mm resulting in an 445

10 Slight Neutral Slight Beneficial Moderate Beneficial Large Beneficial increased risk of flooding to residential properties. Where the proposal may result in a degradation of the water environment because it results in a predicted slight impact on one or more attributes. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: An increase in peak flood level (1% annual probability) > 10mm resulting in an increased risk of flooding to a fewer than 10 industrial properties. Where the net impact of the proposals is neutral, because it results in no appreciable effect, either positive or negative, on the identified attributes. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: Negligible change in peak flood (1% annual probability) < +/- 10mm. All other situations where the proposal provides an opportunity to enhance the water environment or provide an improved level of protection to an attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: A reduction in peak flood level (1% annual probability) > 10mm resulting in a reduced risk of flooding to residential properties. Where the proposal provides an opportunity to enhance the water environment, because it results in a moderate improvement for an attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: A reduction in peak flood level (1% annual probability) > 10mm resulting in a reduced risk of flooding to > 100 residential properties OR a reduction of > 50mm resulting in a reduced risk of flooding to residential properties. It is extremely unlikely that any proposal incorporating the construction of a new or improved trunk road would fit into this category. However, proposals could have a large positive impact if it is predicted that it will result in a very or highly significant improvement to a water attribute(s), with insignificant adverse impacts on other water attributes. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: A reduction in peak flood levels (1% annual probability) >50mm reducing the risk of flooding to >100 residential properties OR a reduction of > 100mm resulting in a reduced risk of flooding to residential properties. Source: DMRB Volume 11 Section 3 Part 10 Table A Baseline Conditions and Sensitivity Baseline Conditions The SBL provides a transport link approximately 4.5 km long between the A370 Long Ashton bypass and AVTM scheme west of Bristol and Hengrove Park in south Bristol. The link includes new and existing highway and a new rapid transit only link to the AVTM scheme. The layout of the proposed route is as shown in Figure The proposed route crosses areas of previously developed and undeveloped land, as shown in Figure Within the undeveloped land, the route passes through the Ashton/Longmoor Brook and New Colliter s Brook flood plain and open farmland, crossing several streams and unnamed drains. The watercourses in the vicinity of the site have historically been subject to diversion and hydraulic improvement. The Malago, Ashton/Longmoor Brook, the New Colliter s Brook and the Old Colliter s Brook are all designated main river watercourses under the jurisdiction of the Environment Agency. The watercourses directly affected by the proposed scheme are the Ashton/Longmoor Brook, Colliter s Brook, New Colliter s Brook and Malago. In addition to these main rivers there are a series of ordinary watercourses and agricultural drainage ditches south of the A38 which will also be affected by the proposed scheme. 446

11 The available information regarding the design and operation of the existing road drainage system suggests a proportion of the route within the developed land is served by existing surface water sewer systems. Most routine runoff is assumed to discharge to surface water Ground investigations undertaken for the route (Structural Soils Ltd, December 2011) established land within the undeveloped sections generally comprise of clay overlain by made ground to a depth of around 1m in some locations. As part of the ground investigation soak away infiltration tests were carried out in four of the trial pits located in the undeveloped land to aid in the design of a sustainable surface water drainage system. However, due to the impermeable nature of the clay, no appreciable drop in water levels occurred during the tests and infiltration rates could not be established. It was concluded that drainage via infiltration methods are not viable options for the route (Halcrow, 2013), which has been accounted for within the Drainage Strategy. Environment Agency Flood Zones Figure 16-1 shows the areas of the proposed route which fall within the Environment Agency Flood Zones, and the following table shows the total length of the proposed route falling within the Environment Agency Flood Zones. This demonstrates that whilst the majority of the route lies outside of the floodplain in Flood Zone 1, there are notable lengths within Flood Zones 2 and 3. Table 16.7: Sections of route within the Environment Agency Flood Zones Environment Agency Flood Zone Route length falling within Flood Zone (km) % of total route Flood Zone Flood Zone Flood Zone

12 Colliter s Brook Ashton/Longmoor Brook Malago Figure 16.1: Route alignment and Environment Agency Flood Zones (Light blue = Flood Zone 2, Dark blue = Flood Zone 3) Watercourse Importance The assessment of the importance of each watercourse potentially affected by the proposed route is summarised in Table In terms of pluvial and groundwater flood risk this has been assessed as having a Low importance for both flood sources due to the limited floodplain extents likely from these sources, and the low number of properties potentially at risk The effect the Importance attribute for each watercourse has on the significance of effect is discussed in the next section. 448

13 Table 16.8: Assessment of watercourse Importance Watercourse Assessment of Importance Ashton/Longmoor Brook Very High New Colliter's Brook (downstream of the railway Very High line) Colliter's Brook (upstream of the railway line) Very High Ordinary Watercourses south of the A38 to Low Highridge Common The Malago Very High Identification and Assessment of Likely Significant Effects The potential effects set out in this section do not take into account proposed mitigation measures. Construction Phase The effects of construction have been assessed for particular activities and how they may vary according to different watercourses and their sensitivity. Significance of effects of construction impacts on surface water flood risk, groundwater flood risk and fluvial/tidal flood risk have been considered. The importance of the receiving water feature, magnitude of impact of construction activity (excluding mitigation) and the significance to the water environment is reported Table 16-9 summarises the magnitude of impacts and significance for the watercourses impacted by the construction phase of the proposed Scheme. This indicates that there are potentially Very Large impacts on fluvial flood risk as a result of the construction phase of the proposed Scheme Table summarises the magnitude of impacts and significance of effects on surface water and groundwater flood risk from the construction phase. The main construction risks to groundwater flood risk would be encountering pockets of water existing in the ground whilst cutting into the ground. Due to the Low Importance classification of surface water and groundwater flood risk the significance of impact is considered to be Slight The influence of the proposed mitigation measures to reduce these impacts will be discussed in the subsequent section. Table 16.9: Assessment of construction phase fluvial/tidal flood risk impacts Construction Activity Watercourse Description of potential effects Importan ce Magnitude of Impact Significance Channel diversion and/or blockage whilst constructing new bridge crossings. Longmoor Brook and Colliter s Brook Blockage or diversion works required during construction of bridges may increase flood risk upstream or downstream. Very High Major Very Large New bridge crossing and bank raising. Longmoor Brook New bridge crossing has been designed as clear span bridge with a soffit level >0.6m above the 1% AEP event (plus an allowance for climate change). It does not therefore affect channel conveyance. Proposed bank Very High Negligible Neutral 449

14 raising is not located in the floodplain. No change in peak water levels. Construction of raised embankment in the floodplain. New Colliter's Brook (downstream of the railway line) Proposed raised embankment separates the New Colliter s Brook from its natural floodplain, and reduces storage available. Peak water levels increased from 7.95mAOD to 8.17mAOD Very High Major Very Large Channel realignment, bank works and replacement of bridge crossing. Colliter's Brook (upstream of the railway line) Bridge crossing has been designed as a clear span bridge with a soffit level above the 1% AEP event (plus an allowance for climate change). Realignment works will maintain existing channel capacity. Proposals do not therefore affect channel conveyance, flow or water levels. Very High Negligible Neutral Culverting of field drains and ground raising. Diversion of channels. Ordinary Watercourses south of the A38 to Highridge Common Potential for reduction in channel conveyance and loss of floodplain due to ground raising. Proposed culverts will be sized to convey the 1% AEP event (including climate change) with a 0.3m allowance of freeboard. Low Minor Neutral Construction of new road in the floodplain. Additional loads on the existing culvert. The Malago The proposed route does not include any ground raising and crosses the Malago at a point where it is already culverted and will therefore have no impact on flow, level or floodplain extents. Very High Negligible Neutral Table 16.10: Assessment of construction phase surface water and groundwater flood risk impacts Construction Activity Water Feature Area Description of potential effects Importance Magnitude of Impact Significance Discharge from storage and site compounds. All watercourses Potential for additional discharges above the greenfield rates from site and storage compounds due to increased impermeable area. Low Minor Neutral Raised embankment alters natural surface water flow paths. All drainage ditches and watercourses Raised embankment will disrupt natural surface water flow paths and alignment of existing drainage ditches, this may result in localised Low Moderate Slight 450

15 ponding. Construction in perched water table area. Elevated pockets of groundwater above the water table Works may disrupt groundwater and may result in localised ponding. Low Minor Neutral Discharge of water from site. Groundwater Potential for increased discharges to groundwater sources due to increase in impermeable areas. Low Minor Neutral Operational Phase Table summarises the magnitude of impacts and significance for the watercourses impacted by the operational phase of the proposed Scheme. This indicates that the impact on fluvial/tidal flood risk is neutral, with the exception of the New Colliter s Brook, downstream of the railway and the Malago. Table 16.11: Assessment of operation phase impacts on fluvial/tidal flood risk Operation Activity Watercourse Description of potential effects Importance Magnitude of Impact Significance Route Operation Ashton/Longm oor Brook There will not be any significant operational impacts as a result of the operation of the proposed development. Very High Negligible Neutral New Colliter's Brook (downstream of the railway line) Additional loading on the existing Armco culvert could impact on structural integrity and thus limit culvert capacity, leading to an increase in flood risk. Very High Major Very Large Colliter's Brook (upstream of the railway line) Ordinary Watercourses south of the A38 to Highridge Common There will not be any significant operational impacts as a result of the operation of the proposed development. There will not be any significant operational impacts as a result of the operation of the proposed development. Very High Negligible Neutral Low Negligible Neutral The Malago Additional loading on the existing culvert could impact on structural integrity and thus limit culvert capacity, leading to an increase in flood risk. Very High Major Very Large 451

16 Table summarises the magnitude of impacts and significance of effects on surface water and groundwater flood risk of the operational phase. Due to the Low Importance classification of surface water and groundwater flood risk the significance of impact is considered to be Slight. Table 16.12: Assessment of operational phase surface water and groundwater flood risk impacts Operation Activity Water Feature Area Description of potential effects Importance Magnitude of Impact Significance Additional surface water runoff from increased impermeable areas Discharge of water from route. Floodplain / All watercourses Groundwater Potential for increase in surface water runoff as the area of impermeable surfaces is increased. This effect could impact across the length of the route. Potential for increased discharges to groundwater sources due to increase in impermeable areas. Low Low Minor Minor Slight Neutral Mitigation Measures Mitigation measures for flood risk and drainage are required to prevent any increase to flood risk in the area, including taking into account the impact of climate change over the lifetime of the development. The proposals for the Scheme mitigation measures are provided in more detail in the FRA and Drainage Strategy (Halcrow, 2013). The influence of the proposed mitigation measures on flood risk is summarised below The principle aim of the FRA and Drainage Strategy (Halcrow, 2013) is to maintain the existing hydrological behaviour as far is reasonably possible, and to mitigate for detriment in line with the design principles stated in Section The finalised plan for mitigating flood risk will be reviewed and approved by the Environment Agency and LPA s The Drainage Strategy developed by Halcrow (2013) sets out the methodology and mitigation measures proposed to manage surface water as part of the proposed development. The approach set out by Defra and the Environment Agency Flood Risk User Guide has been used to estimate to calculate the permitted discharge rate and associated storage volumes. The criteria state that the discharge flow rate from redevelopment sites should be less than or equivalent to the runoff generated from the existing site in its current state for events up to and including the 1% AEP event, including an allowance for climate change. This criteria has been met with regard to the proposed Scheme. Construction Phase Mitigation Fluvial/Tidal Flood Risk The following measures are proposed to mitigate for fluvial flood risk, where appropriate these have been tested within the hydrological and hydraulic model: New bridge crossings (Colliter s Brook and Longmoor Brook) have been designed as clear span structures with a minimum soffit level above the 1% AEP event (including an allowance for climate change) plus 0.6m freeboard; Road levels are set to a minimum level of 1% AEP event (including an allowance for climate change) plus 0.6m freeboard; A compensatory flood storage area is proposed between the New Colliter s Brook and Longmoor Brook to mitigate for the loss of floodplain storage from the proposed raised road embankment on the left bank of the New Colliter s Brook. This will provide an additional 15,000m³ of storage capacity and result in betterment in flood risk; 452

17 It has been assumed that a phased approach for construction will be undertaken and that storage mitigation will be phased in advance of other construction activity. This would avoid any potential increase in flood risk from construction related activities; The proposed realignment of the Colliter s Brook adjacent the Viridor site has been designed to maintain current channel capacity to prevent any increase in flood risk; Should a flood event of sufficient scale occur during construction then the safety of the staff and users of the service could be ensured with the implementation of a comprehensive Flood Management Plan approved by the Local Planning Authority and Environment Agency; Temporary works consent would be obtained from the relevant drainage authority if construction involves blockage of a channel; No large volumes of construction materials would be stored on the floodplain and plant and materials would be well secured to ensure no impediment of conveyance or blockage of channels; A flow assessment was completed to ensure that the new culverts proposed for the Ordinary Watercourses and drainage ditches south of the A38 would be capable of providing conveyance for a 1% AEP event (including an allowance for climate change), with an additional 0.3m freeboard, in line with guidance from the DMRB. New culvert lengths have been minimised where possible; The extension of the existing A38 culvert will ensure the current culvert capacity is maintained; and The detailed design of the proposed realignment of the Colliter s Brook and the new bridge crossings will incorporate ecological and landscape measures in order to maximise the potential for visual and nature conservation benefits in line with the WFD, whilst still allowing for appropriate maintenance Table summarises the impact of the mitigation measures. Table 16.13: Fluvial flood risk mitigation measures and resulting Significance impact Construction Activity Channel diversion and/or blockage whilst constructing new bridge crossings. Construction of raised embankment in the floodplain. Culverting of field drains and ground raising. Diversion of channels. Construction of new road in the floodplain. Watercourse Mitigation Importance Magnitude of Impact Longmoor Brook and Colliter s Brook New Colliter's Brook (downstream of the railway line) Ordinary Watercourses south of the A38 to Highridge Common The Malago Temporary works will be consented and ensure existing channel capacity is maintained and/or mitigation in place such that flood risk will not be increased. Flood relief culverts linking the channel to the floodplain and compensatory storage (15,000m³) provided to prevent detriment. Baseline peak water levels in this reach are 7.95mAOD, decreasing to 7.93mAOD with scheme and mitigation Proposed culverts will be sized to convey the 1% AEP event (including climate change) with a 0.3m allowance for freeboard. The proposed route does not include any ground raising and crosses the Malago at a point where it Significance Very High Negligible Neutral Very High Negligible Neutral Low Negligible Neutral Very High Negligible Neutral 453

18 is already culverted and will therefore have no impact on flow, level or floodplain extents. Surface Water and Groundwater Flood Risk The following measures are proposed to mitigate for surface water and groundwater flood risk during construction: Site runoff would be attenuated and discharged with consent of the relevant drainage authorities and landowners; If required, mitigation of surface runoff from compounds and other temporary surfaces would be agreed with the relevant drainage authority and the Environment Agency; and All temporary surface water detention basins will be located outside of Flood Zone 3 to ensure their continued function under flood conditions Table summarises the impact of the mitigation measures. Table 16.14: Surface water and groundwater flood risk mitigation measures and resulting Significance Construction Activity Water Feature Area Mitigation Importance Magnitude of Impact Significance Discharge from storage and site compounds. All watercourses Discharge will be limited to Greenfield runoff rates using SUDS. Low Negligible Neutral Raised embankment alters natural surface water flow paths. All drainage ditches and watercourses Flow assessments have shown that the proposed new drainage ditches, extended and new culverts will safely convey 1% AEP flows (including an allowance for climate change). Low Negligible Neutral Construction in perched water table area. Elevated pockets of groundwater above the water table Further groundwater monitoring will be undertaken to inform construction methods. Low Minor Neutral Discharge of water from site. Groundwater Discharge will be limited to Greenfield runoff rates using SUDS. Low Negligible Neutral 454

19 Operational Phase Mitigation Fluvial/Tidal Flood Risk The mitigation measures required during the operational phase are limited to the strengthening works required to protect the existing culverts for the New Colliter s Brook and the Malago; the following table summarises this impact. Further details are provided in the appendices of the FRA (Volume 3, Appendix 16.1). Table 16.15: Fluvial flood risk mitigation measures and resulting Significance Operation Activity Water Feature Area Mitigation Importance Magnitude of Impact Significance Additional loadings on the existing watercourse culverts. New Colliter's Brook (downstream of the railway line) Structural assessment of change in loadings completed; proposed that a buried bridge consisting of a concrete slab on bank seats supported by bored piles constructed to protect existing culvert. Very High Negligible Neutral The Malago Very High Negligible Neutral As part of the proposed scheme Environment Agency access to the watercourses for maintenance activities will be maintained and improved where possible, this includes provision of hard-standing areas in some locations for Environment Agency vehicles. Surface Water and Groundwater Flood Risk The following measures are proposed to mitigate for surface water and groundwater flood risk during operation, these are discussed in more detail within the Drainage Strategy (Halcrow, 2013): A range of SUDS measures will be employed (such as detention basins, tank sewers and swales) to ensure that runoff for all new impermeable areas would be attenuated such that it would not cause or increase flooding elsewhere. Discharge from all surface water detention basins would meet Greenfield run-off rates. Mitigation for all new impermeable areas would be in the form of detention basins that would be of a size sufficient to attenuate outfall flow for a 1% AEP event (plus an allowance for climate change); Carriageways will be above known historical groundwater levels to ensure no known groundwater flood risk during operation; and Maintenance of the drainage system would be undertaken by the relevant highways authority, in accordance with their normal practices. Table 16.16: Surface water and groundwater flood risk mitigation measures and resulting Significance Operation Activity Water Feature Area Mitigation Importanc e Magnitude of Impact Significance Additional surface water runoff from increased impermeable areas. Floodplain / All rivers Additional runoff will be attenuated using SUDS, primarily a series of detention basins for the road sections to be constructed in greenfield areas. In urban areas a betterment of 30% from existing will be provided. Low Negligible Neutral Discharge of water from route. Groundwater Attenuation features included in the design to limit discharges to existing rates. Low Negligible Neutral 455

20 16.7. Residual Effects The following section outlines the residual effects and risks to the Scheme, and demonstrates that there are minimal residual effects during construction and operation as a direct result of the Scheme on flooding, leading to a significance category of neutral. Tidal / Fluvial Flood Risk It is possible that a combination of extreme high tides in the River Avon and a major fluvial storm in the Ashton Vale area could result in the Flood Zone 2 outline shown in Figure Flows in the Ashton/Longmoor Brook could become tide-locked when extreme high tides prevent the Brook discharging through the outfall to the river. The outfall is fitted with a tide flap that prevents the incursion of extreme high tide levels. These extreme tide levels are higher than the flood level of the stored fluvial flows prevented from discharging during the high tide window. Storage of these tide-locked flows takes place initially in the lower sections of the catchment downstream and the SBL route would only be affected in the most extreme combined events The detailed modelling undertaken as part of the FRA has indicated that elements of the Scheme adjacent the Ashton/Longmoor and Colliter s Brooks would not be at risk from the extreme 0.1% AEP event, with route levels set at least 0.6m above the water level for the 0.1% AEP event At Queens Road both the existing and proposed carriageway is at risk from flooding in a 1% AEP fluvial event, and would be subject to increased depths at a more extreme 0.1% AEP event. The proposed development does not change ground levels or the proportion of impermeable surfaces; hence the level of risk will be the same as in the existing situation. Surface Water and Groundwater Flood Risk Under extreme events where the SUDS features and drainage ditches proposed in the Drainage Strategy (Halcrow, 2013) are exceeded the route and surrounding areas may be exposed to surface water flooding. The proposed Scheme will not however increase this risk, indeed with the SUDS measures and ditches in place there may be betterment, and flood flows may be directed away from developed areas. Reservoir Flood Risk Background Barrow Reservoir No 3, owned and operated by Bristol Water Plc, is located in the headwaters of the Colliter s Brook The discharge from the overflow relating to an event covered by the Flood Studies Report relating to Barrow No. 3 reservoir would be routed through the compensation reservoir and down through Barrow Gurney. The Dam Category under the Reservoirs Flood and Wave standard is currently Category B and this will not change in the event of a dam breach unless the breach takes water along this route at much greater flows than those considered under the Floods and Wave Standard, which is unlikely. Using the Interim Guide to QRA ICE 2004, the overall consequence class for the current Barrow No. 3 downstream arrangement has been accessed by Bristol Water as A2. Any development or changes to the routing within the downstream zone could increase the level to A1 as the potential loss of life is likely to increase. This would change the Quantitative Risk Assessment and could raise the cost of operating, improving and maintaining the asset The reservoir is subject to regulatory control under the Reservoirs Act 1975 (and the Flood and Water Management Act 2010), in particular reporting under Sections 10 & 11 of the Act. The reservoir is subject to regular inspection including annual inspection by a Supervising Engineer and major 5 yearly inspections by an Inspecting Engineer. The reservoir has been subject to ongoing maintenance and improvement works over many years to ensure that the structure is safe and fit for purpose. As a result of the classification of the reservoir, the inspection regime is in place to ensure that there is an extremely low risk of failure of the reservoir embankment. 1 Ashton Vale to Temple Meads & Bristol City Centre Rapid Transit Scheme Flood Risk Assessment, Ove Arup & Partners, April

21 Consultation with Bristol Water has indicated that although it is unlikely the construction of the SBL will have a detrimental affect on the structural integrity of Barrow Reservoir No 3; Bristol Water recommend that the Reservoir Inundation Model is rerun to determine the potential impact of the SBL on the flood flow routes, depths, velocities etc The need to adhere to the Reservoirs Act is acknowledged. However, the applicant has asked Bristol Water for further evidence to justify the purpose of requesting additional hydraulic modelling to consider changes to the impacts of a dam failure as a result of the implementation of the SBL scheme. This further justification has not been received at the time of printing the Environmental Statement. Reservoir Inundation Modelling Inundation zone modelling of a breach failure of the reservoir embankment (Halcrow Group, 2010), undertaken most recently to assess the risk to the proposed Ashton Park Urban Expansion Area, has indicated that low areas through much of Ashton Vale could be inundated to significant depth, and with high velocities in some areas, within an hour of a breach occurring. The flood inundation modelling is designed to show the worst case scenario based on a sudden and total failure of the reservoir The modelling has assessed a total of six breach scenarios for the Barrow Reservoirs No s 1, 2 and 3. Dependent on the breach scenario flood water is shown to travel down the Colliter s Brook valley, and flow through the Colliter s Brook culvert and Brookgate Public Right of Way underbridge through the railway embankment, flooding downstream to Ashton Vale. Dependent on the breach scenario tested, reservoir flood flows will also flow from the west via Long Ashton onto the SBL route, and not via the Colliter s Brook valley. The flood extents downstream of the railway line in Ashton Vale are broadly similar across the different breach scenarios The areas suffering the highest flood depths and velocities following a failure of the reservoir are the Colliter s Brook valley upstream of the railway line and the agricultural land downstream of the railway between Long Ashton and Brookgate. Flood depths are predicted to be >2m, and velocities >2m/s in these areas. These depths and velocities are indicative and do not represent a situation with the SBL route constructed, there would therefore be some change in depths and velocities as a result of the construction of the scheme. For example, it does not include modelling the impact of the new underbridge through the railway embankment which would provide a new flood flow route downstream to Ashton Vale. The proposed embankments downstream of the railway line are likely to provide some attenuation of flood flows, and may therefore go some way to counter the potential increase caused by an additional underbridge through the railway embankment Given the consistency of flood extents between breach scenarios, and the sheer volume of flood water being released from the reservoirs and already shown to pass through the railway embankment, it is considered unlikely that the proposed SBL route will have a significant impact on the reservoir depths, velocities or hazard in this area. However, it is suggested that as part of the next stage further consultation with Bristol Water is undertaken regarding their concerns on reservoir flood risk Consideration of Likely Cumulative Effects The FRA considered a range of scenarios combining the proposed SBL route with the AVTM and Ashton Gateway (Bristol City Stadium) proposed developments, to test potential mitigation options to ensure that whichever combination of developments (SBL, AVTM and Ashton Gateway) proceed, mitigation options are effective at managing flood risk. This approach also ensured that any opportunities to combine mitigation measures between the development proposals will be explored The testing confirmed that sufficient flood storage can be provided under all scenarios to avoid an increase in flood risk; the significance category is therefore neutral. 457